High-fidelity correction of genomic uracil by human mismatch repair activities.
BACKGROUND
Deamination of cytosine to produce uracil is a common and potentially mutagenic lesion in genomic DNA. U*G mismatches occur spontaneously throughout the genome, where they are repaired by factors associated with the base excision repair pathway. U*G mismatches are also the initiating lesion in immunoglobulin gene diversification, where they undergo mutagenic processing by redundant pathways, one dependent upon uracil excision and the other upon mismatch recognition by MutS alpha. While UNG is well known to initiate repair of uracil in DNA, the ability of MutS alpha to direct correction of this base has not been directly demonstrated.


RESULTS
Using a biochemical assay for mismatch repair, we show that MutS alpha can promote efficient and faithful repair of U*G mismatches, but does not repair U*A pairs in DNA. This contrasts with UNG, which readily excises U opposite either A or G. Repair of U*G by MutS alpha depends upon DNA polymerase delta (pol delta), ATP, and proliferating cell nuclear antigen (PCNA), all properties of canonical mismatch repair.


CONCLUSIONS
These results show that faithful repair of U*G can be carried out by either the mismatch repair or base excision repair pathways. Thus, the redundant functions of these pathways in immunoglobulin gene diversification reflect their redundant functions in faithful repair. Faithful repair by either pathway is comparably efficient, suggesting that mismatch repair and base excision repair share the task of faithful repair of genomic uracil.
Larson
Erik D
ED
Department of Immunology, University of Washington School of Medicine, Seattle, WA 98195-7650, USA. elarson@ilstu.edu
Bednarski
David W
DW
Maizels
Nancy
N
D000225:Adenine; D001402:B-Lymphocytes / Q000378:metabolism; D002457:Cell Extracts; D002460:Cell Line; D002467:Cell Nucleus / Q000201:enzymology; D053843:DNA Mismatch Repair; D004270:DNA, Circular / Q000378:metabolism; D004268:DNA-Binding Proteins / Q000378:metabolism; D015894:Genome, Human / Q000235:genetics*; D006147:Guanine; D006801:Humans; D009154:Mutation / Q000235:genetics; D009692:Nucleic Acid Heteroduplexes / Q000378:metabolism; D013379:Substrate Specificity; D014498:Uracil / Q000378:metabolism*; D051981:Uracil-DNA Glycosidase / Q000378:metabolism
D002457:Cell Extracts; D004270:DNA, Circular; D004268:DNA-Binding Proteins; C094443:G-T mismatch-binding protein; D009692:Nucleic Acid Heteroduplexes; D014498:Uracil; D006147:Guanine; D051981:Uracil-DNA Glycosidase; D000225:Adenine